This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. A key component of innate immunity is the production of type I interferons (IFNs), a family of cytokines designed to limit viral replication as well as virus dissemination in vivo. To elicit antiviral IFN responses, mammalian hosts have evolved a variety of pattern recognition receptors (PRRs), that detect nucleic acids or other conserved structural components of viral pathogens and subsequently initiate signaling cascades leading to IFN induction. Melanoma differentiation-associated gene 5 (MDA5) has emerged as a key cytosolic sensor for the detection of various RNA viruses, including picornaviruses and certain Flaviviruses. In addition, it has been recently shown that MDA5 is also critical for the host innate immune response to paramyxoviruses in vivo. Tight regulation of immune signaling pathways is essential for a successful immune response to viral infections. Whereas positive regulatory mechanisms lead to the rapid activation of IFN signaling upon viral infection, negative regulatory mechanisms are required to prevent unwanted or excessive IFN production. Thus, the goal of the proposed study is to better understand the regulatory networks in innate immunity with a specific focus on the viral RNA sensor MDA5. The overall hypothesis is that MDA5 antiviral activity is regulated by cellular proteins as well as by virus-encoded factors for viral immune evasion.